Why Are the Top Methyl Groups Not Chemically Equivalent?

[silversurf]

Homework Statement

I attached a picture of the molecule. My question is why are the methyl group at the top not chemically equivalent. The answer says that this molecule makes three singlets in HNMR but I was thinking it should make only 2 because the top methyl groups are chemically equivalent aren't they?

Homework Equations

The Attempt at a Solution

 

[lurflurf]

They are not equivalent because each of the methyl groups are in different positions (ie different distance from the oxygen). Also they cannot rotate.

 

[chemisttree]

The distance of the two bridge methyls are certainly different distances (through space) from the oxygen but the largest effect is due to the position of one of the methyls in the deshielding zone of the pi system.

 

[silversurf]

Oh I see. Thank you so much!